class NumArray3 {

    private int[] sum; // sum[i]存储前i个元素和，sum[0] =0
                        // sum[i]存储num[0....i-1]的和

    public NumArray3(int[] nums) {
        if(nums.length>0){
            sum = new int[nums.length + 1];
            sum[0] = 0;
            for(int i=1;i<sum.length;i++){
                sum[i] = sum[i-1] + nums[i-1];
            }
        }
    }

    public int sumRange(int i, int j) {
        return sum[j + 1] - sum[i];
    }
}

/**
 * Your NumArray3 object will be instantiated and called as such:
 * NumArray3 obj = new NumArray3(nums);
 * int param_1 = obj.sumRange(i,j);
 */